A loudspeaker is a device which converts an electrical signal into an acoustical signal (i.e., sound) and directs the acoustical signal to one or more listeners. In general, a loudspeaker includes an electromagnetic transducer which receives and transforms the electrical signal into a mechanical vibration. The mechanical vibrations produce localized variations in pressure about the ambient atmospheric pressure; the pressure variations propagate within the atmospheric medium to form the acoustical signal. A horn-type loudspeaker typically includes a transducer assembly, an acoustical transformer and an acoustical waveguide or horn. The transducer assembly may include a cone-type driver 10 as shown in the sectional view of FIG. 1, wherein a voice coil 12 receives the electrical signal via input terminals 14 and produces mechanical vibrations in an annular cone 16 as a function of the electrical signal. The cone-type driver 10 further includes a dust cap 18 attached to and covering the voice coil 12. Consequently, the voice coil also produces mechanical vibrations in the dust cap 18. The cone-type driver 10 is typically constructed to be symmetric about a central axis 20. The majority of the acoustical signal is radiated from the cone, with contribution from the cone periphery 22 and the dust cap 18.
An acoustical transformer 26 (alternately known as a phase plug) is typically disposed adjacent to the cone 16 as shown in the sectional view of FIG. 2. The purpose of the acoustical transformer 26 is to reduce the volume of the air chamber driven by the cone-type driver 10. Without the acoustical transformer 26, the mechanical reactance of the acoustical system facing the driver 10 is high, only permitting mechanical vibrations in the cone 16 and dust cap 18 at lower frequencies. As the frequency of the mechanical vibrations increases, the high mechanical reactance damps the vibrations, thus reducing the radiating efficiency of the driver. The presence of the acoustical transformer 26 creates a reduced air volume chamber 28. This in turn reduces the mechanical reactance, thus allowing mechanical vibrations of the cone 16 and dust cap 18 at higher frequencies.
An acoustical waveguide (or alternately a horn) receives the acoustical signal radiated by the driver and acoustical transformer and directs the signal in a particular direction. In general, the transmission pattern of the driver/impedance matching assemblies is larger than the region bounded by the horn. The horn tends to constrict the transmission pattern of the driver/acoustical transformer, thus increasing the directivity of the overall loudspeaker.
A typical prior art cone-type driver 10 exhibits a disadvantage in that sound radiated from the cone periphery 22 is not coherent in time with the sound radiated from the dust cap peak 24. This is true because the path length from the voice coil 12 to the cone periphery 22 is longer than the path length from the voice coil 12 to the dust cap peak 24. Although the time difference may be only a few microseconds, it is enough to color the resulting acoustical signal radiated from the driver 10 such that the acoustical signal is not a true representation of the original acoustical source. A further disadvantage of the prior art driver 10 is that the presence of its phase plug substantially blocks all but the periphery of the horn throat, producing what is essentially a "ring radiator." As the frequency of the driver 10 increases, the acoustical output becomes more directional. Although any radiator will exhibit this effect to some extent, the directionality of a ring radiator increases more rapidly with respect to frequency than a direct radiating cone-type driver.
It is an object of this invention to provide a loudspeaker system that substantially overcomes or reduces the aforementioned disadvantages while providing other advantages which will be evident hereinafter.